# Trig identity proof

• January 18th 2010, 06:28 AM
Tweety
Trig identity proof
Use the indenities; $(sinA+sinB)$ and $(cosA+cosB)$

to prove that $\frac{sin2x+sin2y}{cos2x+cos2y} = tan(x+y)$

I am really stuck on this, as the question is not clear to me. Is it talking about the addition formula? sin(A+B).
• January 18th 2010, 06:41 AM
red_dog
$\sin A+\sin B=2\sin\frac{A+B}{2}\cos\frac{A-B}{2}$

$\cos A+\cos B=2\cos\frac{A+B}{2}\cos\frac{A-B}{2}$
• January 18th 2010, 06:46 AM
Tweety
Quote:

Originally Posted by red_dog
$\sin A+\sin B=2\sin\frac{A+B}{2}\cos\frac{A-B}{2}$

$\cos A+\cos B=2\cos\frac{A+B}{2}\cos\frac{A-B}{2}$

Oh I am suppose to be using those formulas, got it now!

Thank you.